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Soil-Saprolite Profiles Derived from Mafic Rocks in the North Carolina Piedmont: II. Association of Free Iron Oxides with Soils and Clays¹

ABSTRACT

The association of free Fe oxides with soils and clays from two Enon sandy loam (Ultic Hapludalfs, fine, mixed, thermic) soil-saprolite profiles was studied. Goethite was the dominant Fe oxide identified. Lepidocrocite was detected in trace amounts in some samples. Fe^CBD/clay ratios were highest in the epipedons of these soils suggesting the concentrating of Fe oxides as a result of aluminosilicate mineral weathering. External (BET-N₂) surface area measurements of non-deferrated and deferrated clays were analyzed in conjunction with electron micrographs of selected clay fractions to determine the association of free Fe oxides with aluminosilicate clays as a function of depth in the profile. Free Fe oxides were found to exist mainly as small, discrete clusters in the A and B horizons of both profiles and specific of the clay surface decreased as a result of treatment for Fe removal. However, external surface areas increased for the saprolite (Cr) horizon clays after deferration. One subfraction identified as having an increase in surface area after deferration was fine clay from the Cr2 horizon, Enon (metagabbro) profile. Chemical data and electron micrographs suggest that either partial dissolution of small, poorly crystalline aluminosilicate clays or removal of some Fe or non-Fe oxide aggregating agent results in breakdown of the fine clays into smaller particles of higher net specific surface.

¹ Paper no. 9109 of the North Carolina Research Service, Journal Series, Raleigh, NC 27650.

² Former Graduate Research Assistant and Professors, respectively, Dep. of Soil Science, North Carolina State Univ. Senior author is now Assistant Professor, Soil Science Dep., California Polytechnic State Univ., San Luis Obispo, CA 93407.

Received for publication February 23, 1984. Accepted for publication August 11, 1984.